The VSWR for the 200 to 300-Ohm mis match will be 1.5:1 assuming that is actually what will be presented at the end of your ladder line.

In actuality, depending upon the specific conditions and the frequency of operation, just about any impedance can appear at the feedpoint of the ladderline. While a low impedance may be present, it is more likely that the impedance will be greater than 50-Ohms, perhaps as much as a several K.

Odds are that the 4:1 balun will provide the closest match. While a greater ratio balun could be used, as the ratios get higher keeping the output balanced becomes far more difficult.

I often see guys saying you need a 4:1 balun when you change from 50 ohm coax to ladder line or twin lead. Some how this seems wrong. If twin lead is 300 ohm should the balun be 6:1? Secondly if you were doing the same coax to 450 ohm ladder line shouldn't the balun be 9:1?

So maybe some the kind and nice folks out there cab explain to me why every one says use a 4:1 balun.

If the 300 ohm twinlead really presented a 300 ohmimpedance at the junction point, then a 6 : 1 balun couldbe used. But that happens only when the antenna impedanceat the other end of the feedline is 300 ohms, and thatisn't very often with the antennas most hams use.

Generally, we'll take a doublet or a loop antenna of somesort - without concern for the impedance at the operatingfrequency - and connect it to a balanced feedline. Asa result, the impedance at the bottom of the feedline isanyone's guess. (You can calculate it if you know theimpedance of the antenna and the length of the feedline.)

So the question is, why do folks recommend using a 4 : 1balun with random impedances. The higher the impedanceratio a balun has, the more difficult it is to get it toactually work like a balun. Especially when the load impedanceis something other than the design impedance. So thechoice really comes down to a 4 : 1 or a 1 : 1 balun inthis application. Because of the range of impedancesto be expected in a wire doublet antenna and how thoseimpedances would get transformed in the feedline, theload impedances are generally above 50 ohms, so the4 : 1 balun is more likely to be a closer match fora majority of the loads encountered. (For lower impedanceloads, a 1 : 1 balun would be better.)

The key piece is that the impedance seen by the balunis NOT the same as the characteristic impedance of thefeedline in most ham antennas using twinlead or ladderline.

I wondered about this myself some time ago. A lot of the science is lost in common usage.

The 4:1 balun is the 2nd easiest to wind on a single toroid. The easiest to wind is a 1:1 balun. Being easy to wind they are lower cost and etc. The 9:1 needs to be on a rather large diameter toroid or on three toroids. The 4:1 balun is usually a series wound toroid using 2 individual wraps of bifilar wire with Teflon insulation which is about 100 ohms for each bifilar set. In series it gives 200 ohms and in parallel it gives 50 ohms (output and input of the balun) hence 4:1. Remember the voltage ratio is only 2:1. See various ARRL books on baluns.

The 4:1 balun makes a good middle of the road matching device and will allow for random lengths of ladder line.

A ladder line at 450 ohms feeding a 75 ohm resonant dipole will reflect the impedance of the antenna if the length of the ladder is in ½ wave length increments (remember to use the velocity coefficient). This implies the use of a 1:1 balun at the TX end of the feed line. The VSWR should be 1.5 if the frequency is at or near resonance. A 1.56:1 balun will give close to VSWR=1.

Copyright 2000-2018 eHam.net, LLC
eHam.net is a community web site for amateur (ham) radio operators around the world.
Contact the site with comments or questions.
WEBMASTER@EHAM.NETSite Privacy Statement